Borehole inclinometer tools are frequently used in drilling operations to record the inclination and azimuth of a wellbore section. The location of the wellbore and its compass heading can be determined during drilling operations by periodically measuring the inclination and azimuth of the bottom of the borehole. Such measurements may be taken at 500 feet intervals in a substantially vertical wellbore and may be taken every 30 to 50 feet in a highly deviated well.
A number of tools have been developed to record the inclination and azimuth of a wellbore. U.S. Pat. No. 1,786,184 to Woodmansee discloses a wellbore tool having a cage which is rotatable about a vertical axis defined by two trunnions. A pendulous body supported by an inner frame pivots about a horizontal axis defined by two pintles. The pendulous body has a longitudinal axis which is aligned with vertical. The lower end of the pendulous body moves along an arc with a center defined by the horizontal axis through the pintles. As the longitudinal axis of the tool is displaced from a vertical orientation, the pendulous body remains in a vertical position as the center of mass of the pendulous body pivots about the pintles and causes the cage to rotate about the vertical trunnions. During drilling operations, the tool is lowered into the wellbore and a steel ball is dropped into the drilling mud to trigger a mechanical latch. The latch lowers the pendulous body into contact with a saddle until a braking disk contacts the lower end of the cage. The inclination of the tool is determined by measuring the angle between the longitudinal axis of the pendulous body and the longitudinal axis of the tool. The azimuth of the tool is recorded by the position of a compass needle which is mechanically locked following contact between the pendulous body and the saddle.
The tool disclosed in U.S. Pat. No. 2,770,887 to Barnett et al. uses a pendulous indicator assembly which is mounted above the upper end of a stem. A marking tip is connected to the upper portion of the indicator assembly. The stem is spring loaded at its lower end to cushion the weight of the indicating assembly. As the longitudinal axis of the tool is displaced from the vertical, the indicator assembly pivots about the stem so that the marking head points vertically upward. Concurrently, a magnet rotatably orients the marking head to indicate the azimuth of the tool. At a predetermined time, a timer actuates a mechanism to lower a metal or paper marking chart into contact with the marking head. The chart is then withdrawn from contact with the marking head to prevent damage to the chart as the tool is removed from the borehole. U.S. Pat. No. 2,770,887 discloses another embodiment having a marking chart supported on the convex surface of a pendulous, inner gimbal. The inner gimbal is pivotably connected to an outer gimbal which is attached to the tool along an axis defined by two pivots. The chart is marked by lowering an indicating head into contact with the chart. The indicating head is then withdrawn from contact with the chart as previously described.
U.S. Pat. No. 2,879,443 to Abs, a marking chart is set in the concave surface of a compass head which rotates about a pivot. A marking pin is located at the lower end of a pendulum suspended by a modified gimbal. To mark the chart, a timer mechanism raises the compass head until the marking pin penetrates the chart. The timer then lowers the compass head to prevent damage to the chart and to the pendulum.
To increase the accuracy of the inclinometer tools such as those discussed above, inclinometer tools typically use delicate suspension systems such as gimbals mounted on bearings to support a pendulous marking head. However, the delicate suspension systems can be easily damaged by impact shocks and vibrations as the tools are lowered into and retrieved from the borehole. Moreover, severe damage to the tool can occur due to careless handling on the floor of a drilling rig. Damage to the inclinometer tools may increase the possibility of errors in the measurements and may ultimately render the tools inoperable.
Another inclinometer tool not discussed above uses a downhole camera to photograph various sensors in the tool which indicate the inclination and azimuth of the borehole. After the tool is raised to the surface by tripping out the drill pipe or be reeling in a wireline, the film is removed from the tool and is developed. Although the film furnishes a permanent record of the measurements, this tool is not useful in boreholes with excessive temperatures which may damage or destroy the film.
Accordingly, a need exists for an inclinometer tool which accurately measures the inclination and azimuth of a borehole while resisting damage to delicate components of the tool. The tool should be operable in high temperatures and pressures and should be efficiently sealed from the operating environment.